It's really difficult. I have experienced something similar.
I would urge you to stay 'in touch' - however loosely you are able. If you are right - and all things point in that direction - then her partner is counting on people giving up on her due to being pushed away/sidelined/ignored, but she needs outside contact - even if it's just waiting in the wings.
From experience, one thing that did work was just knocking on the door out of the blue and pretending nothing was wrong. A really jolly "Hi Joe - how lovely to see you! Was passing and just came to steal Amy for a quick coffee as she's been bloody impossible to get hold of on the phone!".
This is really hard for 'Joe' to deny and you might get a chance to get her alone and have a chat with her. And if you are able to get her into a conversation, even if she denies everything, there is nothing to stop you giving her some contact numbers and advice. Also telling her that you and her family suspect what is happening and you are all waiting to pick her up if she can find a way to leave...
And even if Joe makes an excuse, you can either put him on the spot and suggest coming in or make a jokey - well I'm round here again Friday so I'll pop back then. Tell Amy she's not wiggling out of it this time!".
I would say though, that if 'Joe' is openly hostile to you, don't push too hard. You don't want your friend to pay the price. But he will at least know that you have his number and this may give him pause.
And of course, you can speak to the police. Coercive control and financial abuse are both against the law.
Only you know the ins and outs of the situation, but just please don't abandon her - however hard she pushes. Thank you for caring enough to post.